1. Field of the Invention
Disclosed herein is a lithium secondary battery and preparation methods thereof, and more particularly, to a lithium secondary battery with improved lifetime and capacity characteristics by suppressing a reaction between propylene carbonate (PC) and carbon as electrolyte constituents.
2. Description of the Related Art
In recent years, according to progress of portable electronic appliances and wireless communication systems, there has been increasing demands for highly reliable, high-performance batteries. Also, for solving problems of energy and environmental requirements, development of secondary batteries as large power sources for realizing electromotive vehicles and effectively utilizing night electricity, has become increasingly demanded. Among these secondary batteries, lithium secondary batteries are drawing particular attention because of their good properties such as high operation voltage and high energy density.
Lithium secondary batteries are classified into lithium-ion secondary batteries using an aqueous electrolytic solution and lithium polymer batteries using a solid-type electrolytic solution. The lithium ion battery has problems, such as leakage of an electrolytic solution while in use. Thus, vigorous research into lithium polymer batteries which are leak-tight and flexibly packaged in a desired shape and can be formed in a large scale has recently been made.
Lithium polymer batteries are expected to solve various problems with lithium ion batteries using a liquid electrolytic solution such as low safety, high cost or disadvantage in attaining large-scale, high capacity batteries. However, in order to solve the above-described problems, lithium polymer batteries must satisfy various requirements such as stability over a wide voltage range so as to withstand overcharge/overdischarge, high ionic conductivity and chemical, electrochemical compatibility with electrode materials or other materials present in a cell.
Japanese Laid-Open Patent Nos. 7-134980, 7-235328 and 9-219188 disclose an anode preparation technique using electrode slurry prepared by mixing a carbon material and a polymer electrolyte in manufacturing an anode. According to this technique, a conduction path of electricity may be closed, the internal resistance of an electrode is disadvantageously high and the energy density per unit area is reduced.
Japanese Laid-Open Patent No. 10-284131 discloses a lithium secondary battery with a path of a gel-type electrolytic solution provided in an active material by adding a polymer to active material powder. In electrodes of the lithium secondary battery disclosed herein, lots of pores are present between electrode active materials, lowering a binding force between the active materials and exhibiting poor battery performance in view of uniformity and distribution of pores.
Japanese Laid-Open Patent No. 8-306353 discloses a technique in which alkali metal containing polymers such as polyethylene oxide (PEO) or polypropylene oxide (PPO) are applied onto electrode surfaces and crosslinked. However, such polymers present several problems, that is, they may be dissolved in an electrolytic solution after crosslinkage because of their own properties and the electrolytic solution may infiltrate into electrodes.
Lithium polymer batteries are classified, depending upon the kind of the electrolyte used therein, into a solid type containing no organic electrolytic solution, and a gel type containing an organic electrolytic solution.
A gel type electrolyte forms a stable gel with a polymer host structure by adding an excessive electrolytic solution. The gel is thereby is impregnated into a polymer matrix. Although the gel type electrolyte has higher ionic conductivity at room temperature than a completely solid polymeric electrolyte, it is rather poor in mechanical characteristic. Examples of an organic solvent useful to form the electrolytic solution include ethylene carbonate (EC) and propylene carbonate (PC). In particular, PC having a high boiling point is essentially necessary for crosslinkage for forming a gel-type polymer electrolyte.
However, PC may be decomposed by reacting with carbon as an anode material to produce a gas, resulting in swelling and deterioration of cycle characteristics.